Comparators are ordinarily made to have high gain so that good limiting will occur even with relatively small signals. The ideal voltage comparator has a reference voltage input and a signal voltage input. When the signal voltage exceeds the reference, the output is a logic one (or logic zero, depending on the inputs used). When the signal voltage is less than the reference, the output is a logic zero (or logic one depending on the inputs used). The input current drain is made as low as possible so as to reduce voltage errors that develop due to the finite internal impedance of the signal voltage source.
In monolithic integrated circuit (IC) design JFET input stages are commonly used to reduce current drain to a minimum. A bipolar transistor input stage requires base current to turn it on whereas a JFET displays only a leakage current that can be made small. However, JFET structures require relatively large IC chip area for good performance and extra processing steps are usually required. Also, while JFET gate leakage current is small at room temperature (about 300.degree. K.), the leakage rises with temperature. At about 400.degree. K. the major advantage of the JFET is substantially reduced.